[ORC-RT] Fix unit tests on Linux.

The unit tests may transiently depend on __orc_rt_log_error, which is usually
provided by an alias when loading the runtime through the JIT. The unit tests
statically link the runtime, so this patch provides a fixed definition.

Author: lhames

compiler-rt/lib/orc/tests/unit/CMakeLists.txt

@@ -2,6 +2,7 @@ set(UNITTEST_SOURCES
   adt_test.cpp
   bitmask_enum_test.cpp
   c_api_test.cpp
+  common.cpp
   endian_test.cpp
   error_test.cpp
   executor_address_test.cpp

compiler-rt/lib/orc/tests/unit/common.cpp

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+/// Defined so that tests can use code that logs errors.
+extern "C" void __orc_rt_log_error(const char *ErrMsg) {
+  fprintf(stderr, "orc runtime error: %s\n", ErrMsg);
+}